28. Thanos
FIRST APPEARANCE: IRON MAN #55 (1973) Thanos is one of the most famous comic book villains in history, having been featured in a multitude of media, and deriving his name from the Greek God Thanatos, who was the personification of death. He is a member of the Titanian Eternals, though his abilities extend far beyond most members of his race due to him being a mutant, granting him not just super-strength and speed far in excess of anyone else, but the ability to manipulate cosmic energy as well as telekinesis and telepathy. As if this wasn't enough, he's also an extremely skilled hand-to-hand fighter, a keen military strategist, and has a genius-level intellect, with a knowledge of science that we as humans cannot even begin to comprehend. Oh, and his transportation chair allows him to travel through space, time and even parallel universes. He's pretty important, then. He appeared briefly at the end of The Avengers movie, and will reportedly be appearing in next year's Guardians of the Galaxy in some capacity.
27. Mystique
FIRST APPEARANCE: MS. MARVEL #16 (1978) Mystique aka Raven Darkholme is one of the most instantly recognisable X-Men characters, a blue-skinned, yellow-eyed shapeshifter who typically aligns herself with the Brotherhood of Mutants, and performs assassinations of powerful humans in their name. She can alter her voice and appearance to exactly replicate anyone (though cannot assume a mutant's powers if she "copies" them), and due to the constant replenishment of her cells, she retains an extremely youthful appearance in spite of being over 100 years old. Various changes to the character throughout the years have included adapting her body depending on the situation she finds herself in (i.e. body armour protruding from her skin, wings and so on), and the eventual reveal that she is the mother of Nightcrawler (surprise, surprise). She was played by Rebecca Romijn-Stamos in the first three X-Men movies, and a younger version is now played by Jennifer Lawrence.
26. Scarecrow
FIRST APPEARANCE: WORLD'S FINEST COMICS #3 (1941) In his day to day life, Scarecrow goes by the name of Dr. Jonathan Crane, a psychologist who uses drugs and psychological mind games to try and evoke fear in his enemies, specifically Batman. His primary weapon is a Fear Gas, a psychotropic compound which makes the victim's fears come to life, and Crane enhances the terror by wearing his iconic Scarecrow mask (which also asks as a gas mask for Crane). He is certainly not among Bats' most statuesque enemies (and so Cillian Murphy was perfectly cast in the part in Christopher Nolan's Batman movies), but he has proven himself to be a surprisingly skilled fighter, even if his forte really is with winning the battle before it even starts. However, Crane's own exposure to fear gas has left him fearing only Batman, but also being addicted to that fear, and as such he constantly finds himself drawn to the Caped Crusader. For sure, one of the creepiest enemies in Bats' rogues gallery.